Two suicide bombers have killed at least 27 people at a Shia mosque in south-eastern Iran, officials say.
The first bomber blew himself up at a checkpoint outside the Jamia mosque in the city of Zahedan, with a more deadly second blast moments later.
Worshippers and Revolutionary Guards were reportedly among the dead.
Regional media reported that a Sunni rebel group, Jundullah, said it carried out the attacks in revenge for the hanging of its leader by Iran in June.
Iranian Health Minister Marzieh Vahid Dastagerdi told the news agency ISNA that the death toll was likely to rise, as 11 of the 270 injured people were in critical condition.
According to reports, the clerical leadership in Iran accused the US of backing Jundullah in order to create instability in Iran. Washington denied the charge.
Senior lawmaker Alaeddin Boroujerdi said the US should be held accountable for the "terrorist acts in Zahedan" because of its support for Jundullah, the official IRNA news agency reported.
An e-mail purporting to be from Jundullah - which has attacked Zahedan before - said the attacks were in response to the execution of Abdolmalek Rigi.
The blasts came as worshippers celebrated the anniversary of the birth of Imam Hussein, grandson of the Prophet Muhammad.
The second blast proved more deadly as it struck people who had gathered to help the wounded.
"The [first] attacker, dressed in women's clothing, was trying to get in the mosque, but was prevented," local MP Hossein Ali Shahriari told Fars news agency.
"When people came to rescue those hit in that blast, another bomber blew himself up. Three to four have been killed at least in the first attack."
US Secretary of State Hillary Clinton condemned the "horrific attack" and called for the perpetrators to be held accountable.
"This attack, along with the recent attacks in Uganda, Pakistan, Afghanistan, Iraq, and Algeria, underscores the global community's need to work together to combat terrorist organisations," she said.
Zahedan is the capital city of Sistan-Baluchistan province, which borders Afghanistan and Pakistan and is prone to unrest.
The BBC's Tehran correspondent Jon Leyne says there is resentment in the mainly Sunni area against perceived discrimination by Iran's Shia establishment.
The region is also a key drug-smuggling route from Afghanistan to Europe, with heavily armed convoys of smugglers adding to the atmosphere of lawlessness in this remote region, our correspondent says.